Hi there,
Are you sure it's transconductance you are looking for?
I may be wrong but I have never heard of a diode having transconductance...
Transconductance is, by definition, a current derivated by the change of voltage at constant bias / static conditions.
Basically, it's a change of output current caused by change of input voltage....
So maybe derivative of I at classic IV characteristics at constant light intensity? Or the other way around.... derivative of current by change in light intensity at constant voltage bias.
Either way I would not call it transconductance...
Shlooky.